The invention relates to an automatic spray gun for atomizing a product such as an abrasive paint, a lacquer, or an enamel, which is water-soluble or dissolved in a solvent.
The invention is especially useful for high-pressure and high-frequency atomization of paints that can be cross-linked by ultraviolet radiation.
This type of automatic spray gun comprises a pressurized propellant gas chamber supplied with pressurized propellant gas, generally compressed air, and a product chamber that is supplied with the product to be atomized under pressure.
A nozzle for atomization of the product to be atomized under pressure is connected to the product chamber. The product is supplied upstream from the spray gun and emerges through a calibrated opening of the nozzle.
Moreover, within the spray gun, there is a slide that passes through the gas and product chambers and that is provided on its front end with a needle valve that can block said nozzle.
The opening of the needle is triggered by the action of the propellant gas, generally compressed air.
On its back end, the slide is made integral with the actuating means, generally a pneumatic cylinder that can move in translation to block or clear said nozzle. The pneumatic cylinder comprises a piston returned to the closed position by a spring and moved into the open position by the propellant gas.
To prevent the propellant gas from travelling from the gas chamber to the product chamber along the slide and conversely to prevent the product to be atomized from travelling from the product chamber to the gas chamber, there are sealed oscillating means crossed by said slide.
During atomization of the product, the propellant gas, intended for atomization, facilitates atomization of the product through the central opening of the nozzle and forms a concentrically oriented jet formed by peripheral air vents.
In such a known automatic spray gun, said slide slides each time that it is necessary to open and close the nozzle.
In the known manner, there is an offset for opening the atomization air before discharge of the product and for ensuring the blocking of the nozzle before the closing of the atomization air.
On the nozzle side, the surface of the slide is in contact with the product that is to be atomized and that is often abrasive and/or corrosive, as is the case, for example, for enamel.
The sealing means can be static or deformable: the static means can be O-ring seals, scraper seals, or catches similar to braces, whereas the deformable means can comprise a bellows, a membrane or other flexible surface mounted between a fixed part and a movable part of the spray gun.
Document FR 2 863 512 thus describes an automatic atomizing spray gun comprising a flexible bellows on the side of the chamber of the product to be atomized. This flexible bellows is attached to the body of the spray gun on its back end and comprises a movable front end that forms a seal around the needle.
The needle comprises a shoulder that works with the seal formed by the front end of the bellows.
The spray gun of document FR 2 863 512, however, comprises a bellows with coils, of which the surface is difficult to rinse and clean, in particular in the depressions and corners between the coils.
Document EP1063018 likewise discloses an automatic spray gun with a membrane in which the needle valve is integral with the membrane and crosses it, in such a manner as to be connected to a pulsation control element.
However, due to the significant surface area of the membrane in contact with the product, the membrane must be protected by a coating, for example by a polytetrafluoroethylene layer.